A majority of current drug therapies are delivered systemically and treat the whole body even when disease is confined to a specific organ or tissue type; this increases the required dosage while causing unwanted side effects such as offsite or systemic toxicity. Further, maintaining a therapeutic dose can be difficult when drugs are quickly metabolized or poorly soluble. Achieving a therapeutic dose with conventional medication over an appropriate time frame requires taking multiple doses, resulting in increased cost and often decreasing compliance from patients. Nanoparticle mediated drug delivery (NMDD) has the potential to address many of these shortcomings. Nanoparticles have high surface area-to-volume (S/V) ratios allowing for a relatively large drug cargo carrying/display capacity to be achieved with relatively few particles. Improved pharmacokinetics from smaller dosages can be accessed from the enhanced permeability and retention (EPR) effect in the case of tumors or if a targeted, localized, and controlled drug delivery can be incorporated into the same platform. Another exciting aspect of NMDD is the possibility of creating truly “theranostic” nanomaterials. These multimodal devices have may allow for simultaneous monitoring of disease while also allowing for targeted, stimuli-responsive release of poorly soluble drugs. Although potentially transformative, the development of NMDD and related theranostic devices still requires solving a plethora of issues ranging from the initial bioconjugation chemistry used to assemble the structures to accurate targeting and then final clearance from the body and this, in turn, is providing many fertile avenues of research.
A significant effort in developing NPs for biological applications has been in facilitating directed cellular uptake. Although a number of chemical and physical methods are available, for example, use of Lipofectamine or electroporation, one of the most popular methods to achieve cellular uptake still remains the use of cell penetrating peptides (CPPs). These are predominantly based on truncated versions of the TAT peptide derived from the HIV-1 trans-activating regulatory protein and typically contain consecutive repeats of positively charged lysine or arginine residues. Such polycationic peptides have facilitated intracellular delivery of cargo such as drugs, NPs, small chemical molecules, and even large DNA fragments that would otherwise not enter the cell. Although some debate continues, the mechanism by which CPPs accomplish cellular uptake is generally agreed upon. The localized CPP positive charge initially associates with the negatively-charged heparan sulfate proteoglycan (HSPG) of the cellular membrane or similar molecules of the glycocalyx allowing for subsequent collection into nascent endosomes which then transport the attached cargo into the cytosol. The vast majority of materials taken up in this manner remain sequestered within the endolysosomol system, unless a given component or cargo has innate membrane crossing or endosomal escape properties by being lipidic, lipophilic, amphipathic or the like. In terms of specific NP delivery, CPPs have mediated cellular delivery of a myriad of NP types ranging from gold NPs and semiconductor quantum dots (QDs) to softer dendrimers and polymersomes. CPPs, which are generally <3 kDa, have even mediated cellular uptake of QDs decorated with over a 1,000 kDa of protein cargo in the form of a macromolecular light harvesting complex. Significant resources continue to be invested to improve CPP-mediated NP delivery including optimizing given sequences, selecting for new motifs, imbuing them with endosomal escape properties, and providing them with organelle-specific delivery properties.
An important factor that is often not appreciated about CPP mediated cellular delivery of almost every type of NP material is that it requires a significant amount of CPP to be decorated around the NP surface in order for uptake to occur efficiently; i.e., high avidity over a short exposure time with significant intracellular accumulation. Typical ratios utilized can range from 10 to 20 CPPs per NP or even higher. The net result is that a significant portion of a given NP's surface area and especially their cargo-carrying capacity may be given over to accomplish this by directed CPP attachment to the NP surface or its peripheral chemical groups. As opposed to relying on multiple copies of a given peptidyl motif or other targeting molecule to achieve effective cellular targeting or uptake, there is a growing literature describing how displaying multivalent versions of cellular binding moieties or CPPs can significantly increase initial cellular binding and subsequent cellular uptake. For example, Grillaud and coworkers showed that high cellular uptake of a series of polycationic adamantane-based dendrons was dependent upon the synthetic generation number. Gray et al. showed that functionalizing liposomes with specific phage-display derived tetrameric peptides increased targeting to a biomarker displayed on human 112009 lung adenocarcinoma cells. The Neundorf Lab demonstrated that dimerization of a novel CPP derived from an antimicrobial protein resulted in enhanced cellular uptake and drug delivery.
As NMDD, theranostics, and a variety of NP-related technologies mature into biological applications, it is critical to optimize their design parameters and especially their cargo carrying and surface display capabilities. A need exists for improved delivery of nanoparticles.